1. Field of the Invention
The present invention relates to annulus pressure responsive downhole tools. Particularly, the present invention provides an improved design for an annulus pressure responsive downhole tool which eliminates the need for using a large volume of compressible liquid or a volume of compressible gas within the tool to compensate for the volume displaced by a power piston of the tool.
2. Description of the Prior Art
It is well known in the art that downhole tools such as testing valves, circulating valves and samplers can be operated by varying the pressure of fluid in a well annulus and applying that pressure to a differential pressure piston within the tool.
The predominant method of creating the differential pressure across the differential pressure piston has been to isolate a volume of fluid within the tool at a fixed reference pressure. Such a fixed reference pressure has been provided in any number of ways.
One manner of providing a fixed reference pressure is by providing an essentially empty sealed chamber on the low pressure side of the power piston, which chamber is merely fixed with air at the ambient pressure at which the tool was assembled. Such a device is shown for example in U.S. Pat. No. 4,076,077 to Nix et al. with regard to its sealed chamber 42. This type of device does not balance hydrostatic annulus pressure across the power piston as the tool is run into the well.
Another approach has been to provide a chamber on the low pressure side of the piston, and fill that chamber with a charge of inert gas such as nitrogen. Then, when the annulus pressure overcomes the gas pressure, the power piston is moved by that pressure differential, and the gas compresses to allow the movement of the power piston. Such a device is shown for example in U.S. Pat. No. 3,664,415 to Wray et al. with regard to its nitrogen cavity 44. This type of devices does not balance hydrostatic annulus pressure across the power piston as the tool is run into the well.
Another approach has been to use a charge of inert gas as described above, in combination with a supplementing means for supplementing the gas charge pressure with the hydrostatic pressure of the fluid in the annulus contained between the well bore and the test string, as the test string is lowered into the well. Such a device is shown for example in U.S. Pat. No. 3,856,085 to Holden et al. When a tool of this type has been lowered to the desired position in the well, the inert gas pressure is supplemented by the amount of the hydrostatic pressure in the well at that depth. Then, an isolation valve is closed which then traps in the tool a volume of well annulus fluid at a pressure substantially equal to the hydrostatic pressure in the well annulus at that depth. Once the isolation valve has closed, the reference pressure provided by the inert gas is no longer effected by further increases in well annulus pressure. Then well annulus pressure may be increased to create a pressure differential across the power piston to actuate the tool.
Also, rather than utilize a compressible inert gas such as nitrogen within such tools, it has been proposed to use a large volume of a somewhat compressible liquid such as silicone oil on the low pressure side of the piston. Such a device is seen for example in U.S. Pat. No. 4,109,724 to Barrington.
One recent device which has not relied upon either a large volume of compressible liquid or a volume of compressible gas is shown in U.S. Pat. No. 4,341,266 to Craig. This is a trapped reference pressure device which uses a system of floating pistons and a differential pressure valve to accomplish actuation of the tool. The reference pressure is trapped by a valve which shuts upon the initial pressurizing up of the well annulus after the packer is set. The Craig tool does balance hydrostatic pressure across its various differential pressure components as it is run into the well.
Another relatively recent developement is shown in U.S. Pat. No. 4,113,012 to Evans et al. This device utilizes fluid flow restrictors 119 to 121 to create a time delay in any communication of changes in well annulus pressure to the lower side of its power piston. During this time delay the power piston moves from a first to a second position. The particular tool disclosed by Evans et al. utilizes a compressed nitrogen gas chamber in combination with a floating shoe which transmits the pressure from the compressed nitrogen gas to a non-compressible liquid filled chamber. This liquid filled chamber is communicated with a well annulus through pressurizing and depressurizing passages, each of which includes one of the fluid flow restrictors plus a back pressure check valve. Hydrostatic pressure is balanced across the power piston as the tool is run into the well, except for the relatively small differential created by the back pressure check valve in the pressurizing passage.
With most of these prior art devices, there has been the need to provide either a large volume of compressible liquid or a volume of compressible gas to account for the volume change within the tool on the low pressure side of the power piston. This compressible liquid or gas has generally either been silicone oil or nitrogen. There are disadvantages with both of these.
When utilizing a tool which provides a sufficient volume of compressible silicone oil to accommodate the volume change required on the low pressure side of the power piston, the tool generally becomes very large because of the large volume of silicone oil required in view of its relatively low compressibility.
On the other hand, there is a danger in tools that utilize an inert gas, such as nitrogen, as in any high pressure vessel.
Furthermore, most of these prior art tools have required relatively high annulus pressure increases, sometimes as high as 2000 psi, for operation.